homeostasis- chapter 16

Question 1

what is homeostasis

Answer 1

• The maintenance of an internal environment within restricted limits in organisms.
• Ensures that the cells of the body are in an environment that meets their requirements and allows them
  to function normally despite external changes.
• There are continuous fluctuations brought about by variations in internal and external conditions such
  as temperature, pH and water potential.
• Changes occur around an optimum point.
• Homeostasis is the ability to return to that optimum point and so maintain organisms in a balanced
  equilibrium.

Question 2

what is the importance of homeostasis

Answer 2

• The enzymes that control the biochemical reactions within cells, and other proteins, such as channel
  proteins are sensitive to changes in pH and temperature.
• Changes to water potential of the blood and tissue fluids may cause cells to shrink and expand as a
  result of water leaving or entering by osmosis.
• Organisms with the ability to maintain a constant internal environment are more independent of changes in the external environment.

Question 3

what are the 5 stages within any control of a self-regulating system

Answer 3

• The optimum point, the point at which the system operates best.
• Receptor which detects any deviation from the optimum point (i.e. stimulus)
• Coordinator, which coordinates information from receptors and sends instructions to the effector.
• Effector, often a muscle or gland which brings about the changes needed to return the system to the
  optimum point.
• Feedback mechanism, by which a receptor responds to a stimulus created by the change to the system
  brought about by the effector.

Question 4

what is negative feedback

Answer 4

Negative feedback is when the change produced by the control system leads to a change in the stimulus
detected by the receptor and turns the system off.

Question 5

what is positive feedback

Answer 5

Positive feedback occurs when a deviation from an optimum causes changes that result in an even
greater deviation from the normal.

Question 6

apart from positive and negative feedback what else is in the coordination of control mechanisms

Answer 6

• Normally have many receptors and effectors.
• This allows them to have separate mechanisms that each
  produce a positive movement towards an optimum.
• This gives a greater control. It is important to ensure that the information provided by receptors is analysed by the coordinator before action it taken.

Question 7

what are hormones

Answer 7

• Produced in glands, which secrete the hormone directly into the blood (endocrine glands)
• Carried in the blood plasma to the cells on which they act (target cells)- which have specific
  receptors on their cell-surface membranes that are complimentary to a specific hormone.
• Are effective in very low concentrations, but often have widespread and long-lasting effects.

Question 8

what is one mechanism of hormone action known as

Answer 8

as the second messenger model

Question 9

what is the mechanism involving adrenaline

Answer 9

• Adrenaline binds to a transmembrane protein receptor within the cell-surface membrane of a liver cell.
• The binding of adrenaline causes the protein to change shape on the inside of the membrane.
• The change of protein shape leads to the activation of an enzyme called adenyl cyclase. This
  converts ATP to cyclic AMP (cAMP)
• The cAMP acts as a second messenger that binds to protein kinase enzyme, changing its shape and therefore activating it.
• The active protein kinase enzyme catalyses the conversion of glycogen to glucose which moves out
  of the liver cell by facilitated diffusion and into the blood, through channel proteins.

Question 10

what is the role of the pancreas in regulating blood glucose

Answer 10

• The pancreas produces enzymes for digestion and hormones for regulating blood glucose
  concentration.
• The pancreas is made up largely of the cells that produce its digestive enzymes.
• Scattered throughout these cells are groups of hormone-producing cells known as islets of
  Langerhans.

Question 11

what cells of the islets of Langerhans include

Answer 11

• A cell, which are the larger and produce the hormone glucagon.
• B- cells, which are smaller and produce the hormone insulin.

Question 12

what is the role of the liver in regulating blood sugar

Answer 12

• Located immediately below the diaphragm and is made up of cells called hepatocytes.
• It serves a large variety of roles including regulating blood glucose concentration.
• The hormones insulin and glucagon take effect on the liver.

Question 13

what are the three important processes associated with regulating blood sugar

Answer 13

• Glycogenesis- is the conversion of glucose into glycogen.
• Glycogenolysis- is the breakdown of glycogen to glucose.
• Gluconeogenesis- is the production of glucose from sources other than carbohydrate. When its
  supply of glycogen is exhausted, the liver can produce glucose from non-carbohydrate sources
  such as glycerol and amino acids.

Question 14

explain how blood glucose concentration is regulated (falls too low/high)

Answer 14

• If the concentration falls too low, cells will be deprived of energy and die.
• If the concentration rises too high, it lowers the water potential of the blood and creates osmosis
  problems that can cause dehydration and be equally dangerous.

Question 15

what are 3 factors that influence blood glucose concentration

Answer 15

• Directly from the diet, in the form of glucose absorbed following hydrolysis of other carbohydrates
  such as starch, maltose, lactose and sucrose.
• From the hydrolysis in the small intestine of glycogen (glycogenolysis) stored in the liver and
  muscle cells.
• From gluconeogenesis, which is the production of glucose from sources other than carbohydrate.

Question 16

what do the B cells of the islets of Langerhans have

Answer 16

receptors that detect the stimulus of a rise in blood glucose concentration and respond by secreting the hormone insulin directly into the
blood plasma

Question 17

what is insulin

Answer 17

• is a globular protein made up of 51 amino acids.
• Almost all body cells have glycoprotein receptors on their cell-surface membranes that bind
  specifically with insulin molecules.

Question 18

what happens when insulin bind with receptors

Answer 18

• A change in the tertiary structure of the glucose transport carrier proteins, causing them to
  change shape and open, allowing more glucose into the cell by facilitated diffusion.
• An increase in the number of carrier proteins responsible for glucose transport in the cell-surface
  membrane. Low insulin concentrations- the protein from which these channels are made is part of the membrane of vesicles. A rise in insulin results in these vesicles fusing with the cell-surface
  membrane so increasing the number of glucose transport channels.
• Activation of the enzymes that convert glucose to glycogen and fat.

Question 19

what is a result of insulin and receptors binding

Answer 19

blood glucose concentration is lowered in multiple ways

Question 20

how is the blood glucose concentration lowered from insulin (4)

Answer 20

• By increasing the rate of absorption of glucose into the cells, especially in muscle cells.
• By increasing the respiratory rate of the cells, which therefore use up more glucose, thus
  increasing their uptake of glucose from the blood.
• By increasing the rate of conversion of glucose into glycogen (glycogenesis) in the cells of the
  liver and muscles.
• By increasing the rate of conversion of glucose to fat.

Question 21

what does the lowering of blood glucose concentration cause B cells to do

Answer 21

to reduce their secretion of insulin (negative feedback)

Question 22

what do the A cells of the islets of Langerhans detece

Answer 22

a fall in blood glucose concentration and respond by secreting the hormone glucagon directly into the blood plasma.

Question 23

what are glucagon’s actions when the A cells detect a fall in blood glucose concentration

Answer 23

• Attaching to specific protein receptors on the cell-surface membrane of liver cells.
• Activating enzymes that convert glycogen to glucose.
• Activating enzymes involved in the conversion of amino acids and glycerol into glucose
  (gluconeogenesis)

Question 24

what does the raising of the blood glucose concentration cause A cells to do

Answer 24

reduce the secretion of glucagon (negative feedback)

Question 25

what is another hormone bar glucagon that can increase blood glucose concentration

Answer 25

adrenaline

Question 26

how does adrenaline raise blood glucose

Answer 26

- Attaching to protein receptors on the cell-surface membrane of target cells. - Activating enzymes that cause the breakdown of glycogen to glucose in the liver.

Question 27

how do insulin and glucagon act together

Answer 27

antagonistically

Question 28

what is diabetes

Answer 28

Metabolic disorder caused by an inability to control blood glucose concentration due to a lack of the hormone insulin or a lass of responsiveness to insulin.

Question 29

explain type 1 diabetes

Answer 29

- insulin dependant - is due to the body being unable to produce insulin. - It normally begins in childhood. - It may be the result of an autoimmune response whereby the body’s immune system attacks its own cells, in this case the beta cells of the islets of Langerhans. - Type 1 diabetes develops quickly, usually over a few weeks, and the signs and symptoms are normally obvious.

Question 30

explain type 2 diabetes

Answer 30

-insulin dependant - is normally due to glycoprotein receptors on body cells being lost or losing their responsiveness to insulin. - However, it may also be due to an inadequate supply of insulin from the pancreas. - Type 2 diabetes usually develops in people over the age of 40 years. There is, however, an increasing number of cases of obesity and poor diet leading to type 2 diabetes in adolescents. - It develops slowly, and the symptoms are normally less severe and may go unnoticed. - People who are overweight are particularly likely to develop type 2 diabetes. About 90% of people with diabetes have type 2.

Question 31

how is type 1 diabetes controlled

Answer 31

- controlled by injections of insulin. - It is a protein so has to be injected. - Dose of insulin must be matched exactly to the glucose intake. - Blood glucose concentration is monitored using biosensors.

Question 32

how is type 2 diabetes controlled

Answer 32

- is usually controlled by regulating the intake of carbohydrate in the diet and matching this to the amount of exercise taken. - In some cases, this may be supplementary by injections of insulin or by the use of drugs that stimulate insulin production. - Other drugs can slow down the rate at which the body absorbs glucose from the intestine.

Question 33

where are kidneys

Answer 33

- In mammals there are two kidneys found at the back of the abdominal cavity, one on each side of the spinal cord. - Usually surrounded by a thick, protective layer of fat and a layer of fibrous connective tissue.

Question 34

what two important homeostatic roles do kidneys play

Answer 34

involved in excretion and osmoregulation. - Filter nitrogenous waste products out of the blood, urea. - They also help to maintain the water balance and pH of the blood.

Question 35

explain the 7 structures of the mammalian kidney

Answer 35

- Fibrous capsule- an outer membrane that protects the kidney. - Cortex- a lighter coloured outer region made up of renal (Bowmans) capsules, convoluted tubules and blood vessels. - Medulla- A darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels. - Renal pelvis- a funnel-shaped cavity that collects urine into the ureter. - Ureter- a tube that carries urine to the bladder. - Renal artery- supplies the kidney with blood from the heart via the aorta. - Renal vein- returns blood to the heart via the vena cava

Question 36

what are nephrons

Answer 36

- Blood is filtered and then the majority of the filtered material is returned to the blood, removing nitrogenous waste and balancing the mineral ions and water. - Around 1.5 million nephrons in each kidney. - Has a network of capillaries around it which finally lead into a venule and then into the renal vein. This leads to greatly reduced levels of urea but the levels of glucose and other substances are almost the same. Mineral ion concentration has been restored to ideal levels.

Question 37

describe the 5 structures of the nephron

Answer 37

- Renal (Bowmans) capsule- cup-shaped structure that contains the glomerulus, a tangle of capillaries. The inner layer of the renal capsule is made up of specialised cells called podocytes. - Proximal convoluted tubule- coiled region of the tubule after the Bowman’s capsule, found in the cortex of the kidney. Many substances needed by the body are reabsorbed into the blood here. Walls are made of epithelial cells which have microvilli. - Loop of Henle- A long hairpin loop that extends from the cortex into the medulla of the kidney. The descending loop runs down from the cortex through the medulla to a hairpin bend at the bottom of the loop. The ascending limb travels back up through the medulla to the cortex. - Distal convoluted tubule- a series of loops surrounded by blood capillaries. Its walls are made of epithelial cells, but it is surrounded by fewer capillaries than the proximal tubule. - Collecting Duct- a tube into which a number of distal convoluted tubules from a number of nephrons empty. It is lined by epithelial cells and becomes increasingly wide as it empties into the pelvis of the kidney.

Question 38

explain the 4 different blood vessels in the nephron

Answer 38

- Afferent arteriole- a tiny vessel that branches from the renal artery and supplies the nephron with blood. This enters the renal capsule of the nephron then forming the glomerulus. - Glomerulus- a many-branched knot of capillaries where fluid is forced out of the blood. These then recombine to form the efferent arteriole. - Efferent arteriole- a tiny vessels that leaves the renal capsule. It has a smaller diameter than the afferent arteriole so causes an increase in blood pressure in the glomerulus. Blood is carried away from the renal capsule and then branches to form the blood capillaries - Blood Capillaries- a concentrated network of capillaries that surrounds the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule and from where they reabsorb mineral salts, glucose and water. These capillaries merge together into venules (tiny veins) that then merge together to form the renal vein.

Question 39

what 4 stages foes the nephron carry out in its role of osmoregulation

Answer 39

- The formation of glomerular filtrate by ultrafiltration - Reabsorption of glucose and water by the proximal convoluted tubule. - Maintenance of a gradient of sodium ions in the medulla by the loop of Henle - Reabsorption of water by the distal convoluted tubule and collecting ducts.

Question 40

what is the glomerulus supplied with

Answer 40

- with blood by a relatively wide arteriole from the renal artery. - The blood leaves through a narrower arteriole and a result there is considerable pressure in the capillaries of the glomerulus. - Blood is forced out through the capillary wall (acts like a sieve)

Question 41

what forms glomerular filtrate

Answer 41

water, glucose and mineral ions that are squeezed out of the capillary

Question 42

how is the movement of the filtrate out of the glomerulus resisted

Answer 42

- Capillary endothelial cells - Connective tissues and endothelial cells of the blood capillary - Epithelial cells of the renal capsule - The hydrostatic pressure of the fluid in the renal capsule space - The low water potential of the blood in the glomerulus.

Question 43

what are adaptations that could reduce the barriers to the flow of filtrate out of the glomerulus

Answer 43

- Inner layer of the renal capsule is made up of podocytes. These cells have spaces in them allowing filtrate to pass beneath them and through gaps between their branches. - The endothelium of the glomerular capillaries has spaces up to 100nm wide between its cells.

Question 44

how does the reabsorption of glucose and water happen by the proximal convoluted tube

Answer 44

- Around 85% of the filtrate is reabsorbed back into the blood. - Works based on size of the molecule- small ones are removed i.e. urea

Question 45

what are absorptions due for the reabsorption of glucose and water by the proximal convoluted tube (3)

Answer 45

- Microvilli to provide a large surface area to reabsorb substances from the filtrate. - Infolding at their bases to give a larger surface area to transfer reabsorbed substances into blood capillaries. - A high density of mitochondria to provide ATP for active transport

Question 45

what is the process of the reabsorption of glucose and water by the proximal convoluted tube

Answer 45

1. Sodium ions are actively transported out of the cells lining the proximal convoluted tubule into blood capillaries which carry them away. The sodium ion concentration of these cells is lowered. 2. Sodium ions now diffuse down a concentration gradient from the lumen of the proximal convoluted tubule into the epithelial lining cells but only through special carrier proteins by facilitated diffusion. 3. These carrier proteins are of specific types, each of which carries another molecule (glucose or amino acids or chloride) along with the sodium ions. Co transport. 4. The molecules which have been co-transported into the cells of the proximal convoluted tubule then diffuse back into the blood. As a result, all the glucose and most other valuable molecules are reabsorbed as well as water.

Question 46

what happens to the descending limb and ascending limb when the remainder of water is reabsorbed from the collecting duct

Answer 46

- The descending limb, narrow, thin walls that are highly permeable to water. - The ascending limb, wider, thick walls that are impermeable to water.

Question 47

how does water move out of the collecting duct

Answer 47

by osmosis through channel proteins (aquaporins)

Question 47

what is the process of the maintenance of a gradient of sodium ions by the loop of Henle

Answer 47

1. Sodium ions are actively transported out of the ascending limb using ATP. 2. This creates a low water potential in the region the medulla between the two limbs (interstitial region). 3. Water passes out of the filtrate in the descending limb by osmosis into the interstitial space. This water enters the blood capillaries in the region by osmosis and is carried away. 4. The filtrate progressively loses water as you move down the descending limb. 5. At the base of the ascending limb, sodium ions diffuse out of the filtrate and as it moves up the ascending limb these ions are actively pumped out so the filtrate develops a progressively higher water potential. 6. In the interstitial space between the ascending limb and the collecting duct there is high water potential in the cortex and lower in the medulla. 7. The collecting duct is permeable to water so as the filtrate moves down it, water passes out by osmosis into the blood vessels. 8. Water moves out the collecting ducts along its whole length.

Question 48

what is ADH

Answer 48

Antidiuretic hormone (ADH) can alter the number of these channels and so control water loss.

Question 48

explain the adaptation and main role of the distal convoluted tube

Answer 48

- Walls have microvilli and many mitochondria that allow them to reabsorb material from the filtrate by active transport. - The main role is to make final adjustments to the water and salts that are reabsorbed and to control the pH of the blood by selecting which ions to reabsorb.

Question 49

what does the water potential of blood depend on

Answer 49

- on the concentration of solutes like glucose, proteins, sodium chloride and other mineral ions as well as volume of water in the body - A rise in solute concentration lowers its water potential

Question 50

what can it be caused by (a rise in solute concentration lowering its water potential

Answer 50

- Too little water being consumed - Much sweating occurring - Large amount of ions e.g. sodium chloride being taken in.

Question 51

how does the body respond to a fall in water potential

Answer 51

- Osmoreceptors in the hypothalamus detect the fall in water potential. - When the water potential of the blood is low, water is lost from these osmoreceptor cells by osmosis. - Due to this water loss the osmoreceptor cells shrink, a change that causes the hypothalamus to produce ADH (anti-diuretic hormone) - ADH passes to the posterior pituitary gland, where it is secreted into the capillaries. - ADH passes in the blood to the kidney, where it binds to specific protein receptors, which leads to the activation of phosphorylase within the cell. - The activation of phosphorylase causes vesicles within the cell to move to, and fuse with, its cell-surface membrane. - These vesicles contains pieces of plasma membrane that have numerous water channel proteins (aquaporins), this considerably increases their number making the cell-surface membrane more permeable to water. - ADH increases the permeability of the collecting duct to urea, which therefore passes out, further lowering the water potential of the fluid around the duct. - Both of these mean more water leaves the collecting duct by osmosis, down a water potential gradient and re-enters the blood. - Osmoreceptors send impulses to the thirst centre encouraging the individual to drink more water. - Osmoreceptors detect the rise in water potential and send fewer impulses to the pituitary gland. - The pituitary gland reduces the release of ADH and the permeability of the collecting ducts reverts to its former state.

Question 51

what is a fall in solute concentration of the blood then raising its water potential caused be

Answer 51

- Large volumes of water being consumed. - Salts used in metabolism or excreted not being replaced in the diet

Question 52

how does the body respond to a fall in solute concentration of the blood which raises it water potential

Answer 52

- The osmoreceptors in the hypothalamus detect the rise in water potential and increase the frequency of nerve impulses to the pituitary gland to reduce its release of ADH. - Less ADH leads to the decrease in the permeability of the collecting ducts to water and urea. - Less water is reabsorbed into the blood from the collecting duct. - More dilute urine is produced and the water potential of the blood falls. - When the water potential of the blood has returned to normal, the osmoreceptors in the hypothalamus cause the pituitary to raise its ADH release back to normal levels.